1. Field of the Invention
The invention relates to the field of high speed buses, particularly those using flat ribbon cable.
2. Prior Art
There is an ever increasing need in the electronics industry for long, high speed buses, particularly for communicating digital information. Current technology provides numerous such buses. However, in many instances, these buses employ relatively expensive cables, such as co-axial cables and expensive bus driving circuitry and receiving circuitry.
Flat ribbon cables are often used for buses. These cables are relatively inexpensive, moreover, there are numerous cable accessories, such as connectors which are readily available. However, the bandwidth of these cables, particularly as they become longer, is limited.
As will be described, the present invention employs ribbon cables in a high speed bus. Through use of a combination of unique geometries, isolation and circuitry, a greatly increased bandwidth is obtained. By way of example, a bandwidth of 100 MHz is obtainable with cable lengths up to 80 feet and with 16 receiving stations on the bus.
The high speed bus of the present invention may be used in countless applications. For example, the bus may interconnect computers housed in separate cabinets, intercabinet connections or intracabinet connections may be made between a memory and a central processing unit, as well as to a host of other equipment.
The present invention resulted from a development program for improving a bus structure in a computerized private branch exchange (PBX). Specifically, the bus was designed for both intershelf connections and intercabinet connections. For this reason, and also to provide more insight into the invented bus, a specific prior art bus structure shall be discussed, along with the improved bus structure in a PBX environment. It will be apparent to one skilled in the art that the invented bus may nonetheless be used in a plurality of other applications.
In one prior art PBX, time division multiplexed signals are transferred along a multi-conductor bus in parallel during each time slot. For this structure, it is assumed that the data is "in phase" along the entire bus. That is, a data bit originating at any station is expected to be propagated to any receiving station within each of the time slots. This restricts the maximum bit rate of the system to a function of the total cable length. As will be seen in the present invention, to eliminate this particular problem, the data is transmitted serially (burst mode transmission). Each time slot then may be shorter than the propagation time along the cable. To prevent bus arbitration problems, each station or unit is allocated its own "private" transmitting line.
In the prior art structure, receiving circuits were separated from the bus (ribbon cable) by relatively long traces on circuit boards. Also, the connector arrangement added to the total effective parallel admittance as seen from the bus, further degrading performance. The present invention provides unique configurations and circuits which substantially improve this aspect of overall bus performance.